There isn’t a perfect time to start preparing for the viva. You don’t really need to start until after you’ve submitted; apart from that you have to think sensibly about your priorities. How you can add this work to your other commitments?
If you think you can manage all the things you need to do in two weeks, that’s fine. Need a month to space things out? No problem.
But… If you’re not sure, if you’re just leaving things for a while… If you’re waiting for the right time… What are you waiting for?
What’s going to be different? What will make next week better than now?
There’s no danger you might be overprepared if you do something now rather than save it for later.
Far better not to wait, start well, than to leave it and rush.